A gas turbine jet engine includes a number of sets of blades that are mounted on rotational hubs. These include fan blades, turbine blades and compressor blades, all of which rotate at high speed. In use, it is possible, although extremely unlikely, for a blade or part of a blade to become detached from the hub on which it is mounted whilst rotating. This results in the blade impacting its trailing blade and surrounding engine casing at high speed. Due to the mass and speed of the blade, the force at which impact occurs can be very high. It is therefore important that any damage caused by such an impact is not critical. Tests are therefore carried out to evaluate the damage which would be caused by the release of a blade during operation, in order to allow for appropriate design measures to be taken.
A known test method comprises detonating an explosive charge located within the blade, close to the root of the blade, thus causing the blade to be released from the hub. This is done by machining a hole along the camber line of the blade close to the edge of the bedding (i.e. that portion of the hub which retains the root of the blade). An aluminium carrier containing an explosive charge is located in the centre of the hole and detonators are placed either side of the charge towards the leading and trailing edges of the blade. At a precisely pre-determined time, when the blade is rotating on the hub at maximum speed, the explosive charge is detonated. This causes the blade to become detached from the hub, and to impact its trailing blade and surrounding engine casing. The damage caused by the impact can then be evaluated.
Since the purpose of the test is to evaluate the damage caused by the blade, it is important that the explosive release of the blade does not significantly alter the blade's structural integrity so that a “worst-case” result can be achieved. For example, the blade should not be weakened to the extent where its performance is changed nor to the point that it is released prior to the determined time release. The timing of release must be controlled carefully to ensure that images of the event can be recorded.
When the above-described test is used with a metallic blade, the explosive charge detonation results in a crack that propagates linearly through the shortest section of the blade which bears the critical failure load and the blade fails in tension under the centrifugal force. The blade is released almost instantaneously in a controlled manner and a clean failure surface is left at the blade root. The test can be used for both straight- and curved-rooted blades. However, it can be difficult to accurately machine a curved hole for the explosive charge and detonators that are required for a curved rooted blade.
In a composite blade, a number of layers of fibrous material are bonded together by resin. Radially extending fibres carry the centrifugal force whilst axial and diagonal fibres act to bind the structure together. A composite blade is much stronger in the plane (axial/radial) than in the thickness (circumferential) direction. This is due to the relatively strong layers of fibrous material being bonded together by relatively weak resin.
Due to the structure of a composite blade, a number of problems occur when the above described method is used to release a composite blade. In particular, when the blade fails due to the explosion, a linear crack does not propagate through the blade, as happens for a metallic blade. In order to ensure that a composite blade is cleanly detached from the hub almost instantaneously in a controlled manner, a shaped explosive charge must be used that causes enough of the radial fibres to fail under centrifugal force, releasing the blade aerofoil. However, when using explosives, the energy causes the surrounding resin to vaporise, overcoming the weak bonds between composite layers. This means that extensive delamination occurs throughout the blade and the structural integrity of the blade is significantly compromised, which may result in a test result that is less than the desirable “worst case”.
In order to help prevent this delamination from occurring, the blade may be reinforced in the thickness (circumferential) direction using Kevlar stitching in a region above the explosive charge. However, the Kevlar stitching requires a number of small holes to be drilled through the blade which reduces the strength of the blade in the radial direction, and also reduces its resistance to twisting.
EP2206892 proposes machining two charge holes, one from each of the leading and trailing edges of the aerofoil blade root towards the central point of the camber line of the blade, and two detonator holes from the underside of the blade root, each detonator hole intersecting one of the charge holes. A slot extending through the entire thickness (circumferential direction) of the blade is formed in alignment with the intersections between the charge and detonator holes to reduce the radial strength of the blade (by pre-severing some of the radially-extending fibres in the blade) so that during an explosive release test, the blade is immediately released upon detonation of the charges and delamination is prevented in the area above the slot. Delamination is further prevented by Kevlar stitching provided in reinforcement areas above the charge holes.